Method using metal-organic framework compositions

ABSTRACT

Embodiments of the present disclosure include a metal-organic framework (MOF) composition comprising one or more metal ions, a plurality of organic ligands, and a solvent, wherein the one or more metal ions associate with the plurality of organic ligands sufficient to form a MOF with kag topology. Embodiments of the present disclosure further include a method of making a MOF composition comprising contacting one or more metal ions with a plurality of organic ligands in the presence of a solvent, sufficient to form a MOF with kag topology, wherein the solvent comprises water only. Embodiments of the present disclosure also describe a method of capturing chemical species from a fluid composition comprising contacting a MOF composition with kag topology and pore size of about 3.4 Å to 4.8 Å with a fluid composition comprising two or more chemical species and capturing one or more captured chemical species from the fluid composition.

BACKGROUND

Gas/vapor separation represents a large share of processing in oil,petrochemical, nuclear and many other industries. Gas/vapor separationrelated to upstream and downstream of natural gas (NG) processing isoften very complex and challenging, particularly at the stage ofsweetening (removal of acid gases (CO₂, H₂S)), dehydration and BTX(Benzene, Toluene and Xylenes) removal.

The control of greenhouse gases is of societal importance because it iswell recognized that it has a significant impact on climate change.Among all the greenhouse gas emissions, CO₂ has received the mostattention due to the large quantities of man-made emissions in theatmosphere. The CO₂ atmospheric concentration has exploded to reachrecord levels in May 2013 of 400 parts per million (ppm), which is anunprecedented level in human history. For years, the scientificcommunity has focused its efforts to develop different strategies tomitigate the undesirable CO₂ emissions in the atmosphere from industrialactivities (particularly emissions of CO₂ originating from the burningof fossil fuels) and transportation. Therefore, CO₂ Capture and Storage(CCS) and CO₂ Capture and Use (CCU) are recognized as strategies toreduce the emissions of CO₂ from the atmosphere.

Among the handful of technologies that tackle this challenge, cryogenicdistillation and liquid amine scrubbing are the dominant methods.Nevertheless, these technologies are highly energy intensive, hence notcost effective. Adsorption is currently recognized as one of thealternative separation technologies that can address capturing the CO₂challenge, taking into account both technical and cost effectiveness.Therefore, the choice of the suitable adsorbents that drives theseparation process is of prime importance. Many materials have beeninvestigated for their properties to adsorb CO₂ selectively, whichincludes zeolites, carbon based materials and metal-organic frameworks.

SUMMARY

Embodiments of the present disclosure include a metal-organic framework(MOF) composition comprising one or more metal ions, a plurality oforganic ligands, and a solvent, wherein the one or more metal ionsassociate with the plurality of organic ligands sufficient to form a MOFwith kag topology.

Embodiments of the present disclosure further include a method of makinga MOF composition comprising contacting one or more metal ions with aplurality of organic ligands in the presence of a solvent, sufficient toform a MOF with kag topology, wherein the solvent comprises water only.

Embodiments of the present disclosure also describe a method ofcapturing chemical species from a fluid composition comprisingcontacting a MOF composition with kag topology and pore size of about3.4 Å to 4.8 Å with a fluid composition comprising two or more chemicalspecies and capturing one or more captured chemical species from thefluid composition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic representation of a 3D kagomé tillingnetwork (color scheme: Zinc=cyan, Nitrogen=blue, Carbon=grey,Oxygen=red, Hydrogen=white), according to some embodiments.

FIGS. 2A-B illustrate graphical views of (A) Adsorption of CO₂, H₂S, CH₄and N₂ on kag-MOF, (B) isosteric heat of adsorption of H₂O vapour, CO₂and CH₄, according to some embodiments.

FIG. 3 illustrates a graphical view of adsorption of H₂O on kag-MOF at298K after evacuation at 393 K, according to some embodiments.

FIG. 4 illustrates a graphical view of adsorption of H₂S as compared tobenzene on kag-MOF at low pressures (No benzene is adsorbed), accordingto some embodiments.

FIG. 5 illustrates a graphical view of experimental and calculatedpowder X-ray diffraction patterns indicating the phase purity andstability of kag-MOF at different thermal and chemical conditions,according to some embodiments.

FIG. 6 illustrates a graphical view of TGA of kag-MOF, according to someembodiments.

FIG. 7 illustrates a graphical view of comparison of IR spectrum of theas-synthesized kag-MOF vs ligand, according to some embodiments.

FIGS. 8A-D illustrate schematic representations of 2D kagomé tillingnetworks, according to some embodiments.

FIG. 9 illustrates a graphical view of CO₂ adsorption (filledcircles)-desorption (open circles) isotherm on kag-MOF at 273 K(evacuation at 393 K). The inset shows the pore size distributiondetermined using NLDFT, according to some embodiments.

FIG. 10 illustrates a graphical view of variable temperature isothermson kag-MOF after evacuation at 393K, according to some embodiments.

FIG. 11 illustrates a graphical view of CO₂ adsorption isotherms onkag-MOF at 298 K after evacuation at 328, 393 and 433 K, according tosome embodiments.

FIG. 12 illustrates a graphical view of Q_(st) of CO₂ adsorption onkag-MOF after evacuation at 323 and 393 K, according to someembodiments.

FIG. 13 illustrates a graphical view of variable temperature CO₂adsorption isotherms on kag-MOF after evacuation at 323 K, according tosome embodiments.

FIG. 14 illustrates a graphical view of variable temperature CH₄adsorption isotherms on kag-MOF after evacuation at 393 K, according tosome embodiments.

FIGS. 15A-B illustrate a graphical views of (A) IAST prediction forCO₂/N₂, CO₂/CH₄ mixtures, (B) Column breakthrough test for CO₂/N₂:10/90mixture adsorption at 298 K and 1 bar total pressure, according to someembodiments.

FIGS. 16A-B illustrate graphical views of (A) pure component CO₂ N₂, CH₄at 298K (B) Q_(st) of CO₂ and CH₄ adsorption after activation at 55° C.and 120° C., according to some embodiments.

FIG. 17 illustrates a graphical view of Density of CO₂ adsorbed at 298K, according to some embodiments.

FIG. 18 illustrates a graphical view of Variable Temperature PXRDpatterns of kag-MOF, according to some embodiments.

FIG. 19 illustrates a graphical view of PXRD patterns of kag-MOF aftersoaking in different solvents for 24 h as evidence of the stability ofkag-MOF, according to some embodiments.

DETAILED DESCRIPTION

In general, embodiments of the present disclosure describe metal organicframework compositions, and methods of making and using MOFcompositions. In particular, the present disclosure describesembodiments including a highly stable metal organic frameworkcomposition fabricated in water without any other solvent and with alow-cost reagent, such as tetrazolate and zinc salts. This MOF platformis a highly stable, nitrogen-rich MOF made in water for multipurpose usein gas/vapor separations, such as dehydration, CO₂ capture, H₂S removaland BTX sieving from natural gas. The MOFs of the present embodimentscan be synthetized in water without any other solvent, with simpleprocess steps and low-cost chemicals, such as zinc salt and tetrazolatebased ligand. The MOF compositions of the present disclosure include akagomé (kag) topology. The kag-MOF composition is highly stable andexhibits suitable properties for CO₂ from flue gas and natural gas indry conditions. The compositions described herein show an excellentbalance between selectivity, CO₂ uptake and energy for regeneration.Additionally, the compositions include desiccant properties,particularly with a low energy for dehydration. The kag-MOF compositionscan also successfully and substantially perform H₂S/CH₄, H₂S/BTX andacetone/phenol separation and gas dehydration.

Although many N-containing MOFs have been reported, systematic studieson CO₂ capture in concurrent correlation to pore size are scarce.Embodiments here describe efforts to tune/design novel materials for CO₂capture, such as developing several strategies to enhance the CO₂energetics in MOFs. As a first example, rht-MOF-7, fabricated bydeliberate modification of the trigonal core of the parent rht-MOF-1 bya triazine core linked to an amine functional group in between the coreand isopthalate termini, was found to be excellent candidate platformsfor systematic enhancement of CO₂ capture efficiency. The resultingsynergetic effect of N-containing ligand and secondary amine wasreflected by a drastic enhancement of the isosteric heat of CO₂adsorption at low loading (44.7 kJ mol⁻¹ versus 35 kJ mol⁻¹ for theparent rht-MOF-1), albeit with a subsequent sharp decrease, mainlybecause of the quick saturation of the highly energetic sites, butnon-homogeneous, and the occurrence of pore filling which follows.

In a second example, the assembly of rare-earth based (terbium) fcuplatform by utilizing polarized ligands containing tetrazolate andfluoro moieties positioning in a close vicinity of open metal sites, hasresulted in very high CO₂ energetics as the initial Q_(st) at lowloading was 58.1 kJ mol⁻¹. Similar to the rht platform, the amount ofhighly energetic sites was limited and non-homogeneously distributed. Inboth cases, rht and fcu platforms possess quite big cavities (10.4 and17.8 Å in the former and 14.5 and 9.1 Å, respectively), therefore theCO₂ adsorption occurs first on the most active (limited) sites whichsaturate quickly leading in turn to a drastic decrease in selectivitytoward CO₂.

In another example, a reticular chemistry approach aiming to build anextended/contracted series of channel based MOFs constructed withbypiridyne/Cu and pyrazine/Zn 2-D periodic 4×4 square grids pillared bySiF₆ anions, led to the unveiling of an effective adsorption mechanismcombining both thermodynamics and energetics. In fact, the contractedanalogue assembled from zinc (or copper) cation and pyrazine ligand,characterized by channels with small pore diameter of 3.84 Å(SIFSIX-3-Zn), showed high and uniform Q_(st) for CO₂ at 45-52 kJ mol⁻¹throughout the loading, affording unprecedented physical adsorbent withhigh selectivity towards CO₂ at various concentrations. With thisfoundation, embodiments of the present disclosure are then directed to anew MOF material with small-channeled pores, assembled usingnitrogen-rich polarizable ligand.

The kag-MOFs described herein can utilize green-synthesis methods forsynthesizing porous materials, which reduce or eliminate the need forsolvents and corrosive metal salt reagents, reduce or eliminate toxicand/or acidic byproducts, and are scalable to industrial levels. Thefuture deployment of MOF materials at the larger scale for manyapplications such as adsorbents, catalysts, and sensors, among manyothers, require clean, environmentally friendly, easy, cost efficientand scalable synthesis procedures. Solution-based MOF synthesismethodologies, such as the solvothermal method previously discussed,suffer from the need to use toxic and/or corrosive metal salts reagents.In addition to being costly and hazardous, these reagents furthergenerate acid byproducts, which are often susceptible to solvolysis andrequire costly disposal. The need for fresh solvents, enhanced safetyprecautions, and waste disposal greatly increases both the capital costsand production costs of these methods. Low reproducibility furtherprecludes industrial applications of solvothermal synthesis methods.

Metal organic frameworks (MOFs) are a versatile and promising class ofcrystalline solid-state materials that allow porosity and functionalityto be tailored towards various applications.

Generally, MOFs comprise a network of nodes and ligands, wherein a nodehas a connectivity capability at three or more functional sites, and aligand has a connectivity capability at two functional sites each ofwhich connect to a node. Nodes are typically metal ions or metalcontaining clusters, and, in some instances, ligands with nodeconnectivity capability at three or more functional sites can also becharacterized as nodes. In some instances, ligands can include twofunctional sites capable of each connecting to a node, and optionallyone or more additional functional sites, which do not connect to nodeswithin a particular framework. In some embodiments, ligands can bepoly-functional, or polytopic, and comprise two or more functional sitescapable of each connecting to a node. In some embodiments, polytopicligands can be heteropolytopic, wherein at least two of the two or morefunctional sites are different.

Embodiments of the president disclosure describe a metal-organicframework composition comprising one or more metal ions, an organicligand, and a solvent, wherein the one or more metal ions associate withthe organic ligand sufficient to form a MOF with kag topology. The oneor more metal ions may include M²⁺ cations. The M²⁺ cations may includeone or more of Zn²⁺, Cu²⁺, Ni²⁺, Co²⁺. The organic ligand may be aninexpensive organic building block. The organic ligand may include any Ndonor ligand. For example, the organic ligand may include one or more oftetrazole, triazole, derivatives of tetrazole, and derivatives oftriazole. The one or more metal ions may associate with the organicligand to form a metal-organic framework with kagomé topology (kag-MOF).(FIG. 1) In some embodiments, the kag-MOF includes a plurality oforganic ligands. In some embodiments, the kag-MOF is formed in thepresence of a solvent, wherein the solvent includes water or is onlywater. In some embodiments, the kag-MOF may be hydrothermallysynthesized. In other embodiments, the kag-MOF may be highly stable. Inanother embodiment, the kag-MOF may be hydrothermally synthesized andhighly stable.

The kag-MOF of the present disclosure may include any combination of M²⁺cations—including, but not limited to, one or more of Zn²⁺, Cu²⁺, Ni²⁺,and Co²⁺— with any combination of organic ligands—including, but notlimited to, one or more of tetrazole, triazole, derivatives oftetrazole, and derivatives of triazole. In some embodiments, the kag-MOFmay include one of the above-mentioned metal cations or a mixture ofmetal cations, with one of the above-mentioned organic ligands, amixture of organic ligands, a plurality of the above-mentioned organicligands, or a plurality of a mixture of organic ligands.

Embodiments of the present disclosure also include methods of making aMOF composition. The method of making a MOF composition may includecontacting one or more metal ions with an organic ligand sufficient toform a MOF with kag topology. In other embodiments, the method of makinga MOF composition may include contacting one or more metal ions with aplurality of organic ligands sufficient to form a MOF with kag topology.In another embodiment, the method of making a MOF composition mayinclude contacting one or more metal ions with an organic ligand or aplurality of organic ligands in the presence of a solvent. In someembodiments, the solvent includes water or is water only.

Embodiments of the present disclosure further include a method ofcapturing chemical species from a fluid composition. The method mayinclude contacting a MOF composition with kag topology and pore size ofabout 3.4 Å to 4.8 Å with a fluid composition comprising two or morechemical species and capturing one or more captured chemical speciesfrom the fluid composition. The MOF composition with kag topology mayinclude any of the embodiments described above. The chemical speciescaptured by the kag-MOF may include one or more of CO₂, H₂S, H₂O, andacetone. In some embodiments, CO₂ is the captured chemical species froma fluid composition including one or more of natural gas, flue gas,syngas, biogas, and landfill gas. In other embodiments, H₂S is thecaptured chemical species from a fluid composition including one or moreof H₂S, benzene, toluene, and xylene. In other embodiments, H₂O is thecaptured chemical species from a fluid composition including one or moreof a gas, a vapor, and a solvent. In other embodiments, acetone is thecaptured chemical species from a fluid composition including one or moreof acetone, phenol, and C₄ fractions. These embodiments are notlimiting, and additional embodiments are provided below.

The kag-MOF described in these embodiments exhibits high chemical andthermal stability based on the kagomé platform. The particularsynergetic effect of highly dense, homogeneously distributed tetrazoles,zinc cations and small pore size of channels, directed the homogeneousand fairly high interactions with CO₂, in one example. Embodimentsherein confirm the high importance of the synergy between at least twodriving forces, namely N-rich containing ligand and small pore size, toachieve the desired materials for capturing CO₂, H₂S, H₂O, and acetone.Owing to its facile hydrothermal synthesis, stability and structuralproperties, applications may utilize this kagomé platform-based MOF as amembrane for various gas separations based on molecular sieving effect.

The kag-MOF of the present disclosure exhibited high chemical andhydrothermal stability. The synergetic effect of the charges densely anduniformly distributed over the pore channels (FIG. 1) and contractedpore size ranging from about 3.4 Å to 4.8 Å led to quite strong andsteady CO₂ isosteric heat of physical adsorption translated into highlyuniform CO₂ interaction within the framework, a key feature that directsthe efficiency of CO₂ capture using adsorbents in dry conditions (FIG.2). As seen from FIG. 2a , the measurement of H₂S up to 0.1 bar (partialpressure) shows that the steepness of the adsorption isotherm in case ofH₂S is almost similar to CO₂, the H₂S in this case is adsorbed stronglyand reversibly. H₂S adsorbs preferentially over CO₂ using kag-MOF.

The heat of adsorption of CO₂, CH₄ and H₂O shows that H₂O interactsstrongly (58 kJ/mol) with the framework of kag-MOF as compared to CO₂(36 kJ/mol) and CH₄ (12.5 kJ/mol) (FIG. 2b ). Therefore, it is expectedthat kag-MOF could be used also as an efficient gas dehydration agent(FIG. 3) for industrial gases such as natural gas (NG). The bestperforming conventional materials for gas dehydration are 4A and 13Xzeolites. For these zeolites, the energy necessary for reactivation (andsubsequent recycling) is twice as much (ca. 120 kJ/mol) as kag-MOF. Thisfinding is important as dehydration of gases is a very energy intensiveprocess and there is a large need for new dehydration agents to reducethe cost of water removal from industrial gases.

Once the CO₂ containing stream was water vapor and H₂S free, the kag-MOFwas an excellent separation agent for CO₂ removal from flue gas andnatural gas. This was confirmed experimentally using column breakthroughtest performed using CO₂/N₂:10/90 mixture (FIGS. 15A-B). In fact, whileCO₂ was retained in the column for 601 s, the first N₂ signal wasobserved just after 2 s, indicative of the high CO₂ selectivity(520±200).

In light of the high H₂S adsorption capacity and the small channel size(aperture) of kag-MOF, the adsorption of benzene (one of the componentsof the undesirable BTX in natural gas) was studied to evaluate itsequilibrium adsorption properties as compared to H₂S. Interestingly,benzene was shown not to adsorb on kag-MOF. See FIG. 4, for example,illustrating a graphical view of adsorption of H₂S as compared tobenzene on kag-MOF at low pressures, according to an embodiment of thepresent disclosure. This second finding shows that kag-MOF canpotentially be used to sieve H₂S from BTX (Benzene, Toluene, Xylene).

Because of the sieving features of kag-MOF for benzene, it is believedthat kag-MOF will be an excellent agent for the important separation ofacetone from phenol and acetone from C₄.

Embodiments of the present disclosure include using kag-MOF compositionsfor H₂S removal from NG. The particular outstanding properties ofkag-MOF materials in terms of stability, H₂S uptake and selectivity makethis novel MOF suitable for many industrial applications where H₂S needto be removed, particularly NG which is of the extreme importance forthe Kingdom of Saudi Arabia.

Embodiments of the present disclosure include using kag-MOF compositionsfor dehydration. The particular outstanding properties of kag-MOF interms of stability to moisture, H₂O uptake and affinity make theseseries of novel MOFs suitable for many industrial applications wherevarious degree of humidity need to be removed.

As discussed above, embodiments of the present disclosure include usingkag-MOF compositions for CO₂ capture from NG. kag-MOF may be used as anefficient adsorbents for the removal of CO₂ at various concentration(from 1% to 20%) from dry and humid conditions relevant to industrialgases.

Embodiments of the present disclosure include using kag-MOF compositionsfor H₂S/BTX separation. The presence of aromatics such as benzene,toluene and xylene (BTX) as contaminants in H₂S gas stream enteringClaus sulfur recovery units has detrimental effect on catalyticreactors, where BTX form soot particles, and clog and deactivate thecatalysts. BTX removal before the BTX and H₂S containing stream enterthe catalyst bed, could be a judicious solution to this problem. Becauseof the sieving affinity and it extremely high stability, kag-MOF may beused as a separation agent on PTSA (pressure-temperature swing system).

Embodiments of the present disclosure include using kag-MOF compositionsfor acetone/phenol and acetone/C₄ separation. The sieving property ofkag-MOF for benzene inspired to launch a study for the separation ofacetone from Phenol and C₄ fractions. kag-MOF is a potential materialfor this separation.

Example: CO₂ Capture

The novel kagomé metal organic framework (MOF) with small pore size andN-rich linker was synthesized hydrothermally via coordination oftetrazole moieties with zinc cations which shows high chemical andthermal stability, as an example. The synergetic effect of the chargesdensely and uniformly distributed over the pore channels and contractedpore size led to quite strong and steady CO₂ isosteric heat ofadsorption translated into highly uniform CO₂ interaction within theframework, a key feature that direct the efficiency of CO₂ capture usingadsorbents.

MOFs represent a class of porous materials that offer high surfaceareas, permanent porosity and chemical tunability, making thesematerials suitable for adsorbing CO₂. However, designing stable MOFswith high selectivity towards CO₂ adsorption is still a challengingtask. In fact, the affinity to CO₂ compared to other gases (N₂, CH₄, O₂,etc.) is one of the key factors influencing the economy of the CO₂capture process. Hence, it is vital to tailor the functionality and thepore size of MOFs simultaneously in order to design novel materialsspecifically dedicated to CO₂. Intensive efforts are currently investedin the search of new strategies allowing the enhancement of bindingenergies between CO₂ molecules and the framework. (i) Incorporation ofLewis basic sites into MOFs (eg: amino groups), (ii) insertion of openmetal sites and (iii) introduction of various other strongly polarizingfunctional groups are three approaches that are most the frequentlystrategies explored.

Indeed, hydrothermal reaction between Zn(NO₃)₂•3H₂O andtetrazole-5-carboxylate, ethyl ester yields a homogenousmicrocrystalline material. The as synthesized compound was characterizedand formulated by single crystal X-ray diffraction studies asZn₅(HTet)₆(Tet)₃(OH⁻)₇. The purity of the material was confirmed bysimilarities between simulated and experimental powder X-ray diffraction(PXRD) (FIG. 5) and elemental microanalysis. The crystallographicanalysis states that a de-esterification occurs during the reaction. Toconfirm this contention, thermogravimetric analysis (TGA) was carriedout under air (FIG. 6) and the residue was analyzed by PXRD. The TGAshows two main drops corresponding to a total loss of 64.4% which is ingood agreement with the theoretical data (62.1%). In addition, infraredspectroscopy measurements performed on the ligand clearly showed thepresence of the ester group, with the characteristic v_(c=o) band (1735cm⁻¹) and the absence of the v_(c=o) band of esters in the kag-MOFindicating the in situ de-esterification of the ligand during thesynthesis. (FIG. 7).

The compound crystallized in a hexagonal system (space group P63/mmc)and contained two crystallographic independent zinc cations, bothcoordinated to six nitrogen atoms of two independent tetrazole ligandsto form octahedral surrounding. Neutral and deprotonated tetrazoleligands adopt tridentate and tetradentate coordination modesrespectively. The assembly of the 6-connected Zn(1) nodes and neutraltetrazole molecules result in the generation of 2D kagomé layersexhibiting hexagonal channels (FIG. 8(a)). The packing of these layersalong the c axis following the axial-to-axial pillaring, using Zn(2)cations and deprotonated tetrazolates, yield a cationic 3D structure,balanced by hydroxide anions, with an overall kagomé tilling (kag)geometry (FIG. 8). The framework exhibits 1D hydrophobic channels with adiameter of around 3.8 Å (FIG. 1).

FIG. 8 describes (a) projection along (001) of the kagomé layer built upfrom Zn(1) cations and neutral tetrazol molecules; (b) projection along(010) showing the packing of kagomé layers via deprotonated tetrazolemolecules; (c) and (d) projection along (001) and (010) respectively ofthe structure showing the introduction of Zn(2) cations within thestructure.

Initial adsorption studies showed that N₂ and Ar probe the framework ofthe compound as non-porous at 77 and 87 K, respectively. This firstresult confirms, that indeed, the pore windows of this material arenarrow, as was concluded from single crystal diffraction data.Accordingly, CO₂ measurement at 273 K was targeted as a primarymethodology to confirm the permanent microporosity, as evidenced by the(fully reversible) type I CO₂ adsorption isotherm (FIG. 9).

The specific BET surface area was estimated to be 211 m²·g⁻¹ using CO₂.The calculated theoretical total free pore volume was estimated to be0.12 cm³·g⁻¹. The pore size distribution, calculated from CO₂ adsorptiondata at 273 K using non-Local density functional theory (NLDFT), wasestimated to be very uniform, centered at diameter of 3.6 Å (FIG. 9), inexcellent agreement with single crystal diffraction data.

The uniformly distributed, exposed tetrazole combined with the reducedpore size of kag-MOF, are attractive features for evaluating the impactof pore size, shape and functionality on CO₂ adsorption energetics anduptake. Accordingly, CO₂ adsorption experiments were carried out atvarious adsorption and desorption temperatures (FIG. 10) in order tolook closely at the effect of total and partial evacuation of theframework on the CO₂ uptake and energetics. Prior to full adsorptionstudy, optimization of activation protocol showed that kag-MOF can befully evacuated at 393 K and further evacuation at 433 K resulted only aslight increase in the CO₂ uptake, particularly at 1 bar (FIG. 11).

The isosteric heat of CO₂ adsorption, Q_(st), was in turn calculated forthe sample evacuated at 323K, 393 K (FIG. 12). The Clausius-Clapeyronequation was used to process the adsorption data collected attemperatures between 258 and 298 K (FIG. 13). Although a slightdifference was observed in terms of uptake, the trends of the CO₂ Q_(st)as a function of loading were found to be similar after evacuation at328 and 393 K (FIG. 11). It is worth mentioning that the accuracy of theQ_(st) determination was confirmed by the established linearity of CO₂isosters for the entire studied range of CO₂ loadings.

The Q_(st) of CO₂ adsorption is an intrinsic property that dictates theaffinity of the pore surface toward CO₂, which in turn plays a majorrole in determining the adsorption selectivity and the necessary energyto release CO₂ during the regeneration step. As illustrated, kag-MOFexhibits quite high CO₂ affinity over a wide range of CO₂ loading (37.5kJ·mol⁻¹) due to the homogenously distributed strong adsorption sites.In light of the direct relationship of CO₂ and CH₄ adsorption energeticsto CO₂/CH₄ adsorption selectivity, the Q_(st) of CH₄ adsorption wasexplored, determined from variable temperature CH₄ adsorption isotherms(FIG. 14). In contrast to CO₂ energetics, CH₄ exhibits much lower Q_(st)at low loading (12.5 kJ·mol⁻¹) combined with decreasing trend as theuptake increased.

Elevated and uniform CO₂ interactions are the key to improve selectivitytowards CO₂, which is a critical parameter for the effectiveness of gasseparation and purification of important commodities from CO₂ (e.g.,CH₄, N₂, H₂). Analysis of data using IAST (FIGS. 15A-B), shows that thismaterial displays high CO₂/N₂ selectivity (ca. 700 at 1 bar for example)over a wide range of pressure while the calculated CO₂/CH₄ selectivity(88 at 1 bar) was comparatively low. In case of CO₂/N₂ mixture, the lowN₂ adsorption uptake/energetics, translated into low affinity for N₂(FIG. 15A) combined with the relatively strong energetics of CO₂adsorption (FIGS. 16A-B), results in high equilibrium adsorptionselectivity toward CO₂ (FIG. 15B).

This result was confirmed experimentally using column breakthrough testperformed using CO₂/N₂:10/90 mixture. In fact, while CO₂ was retained inthe column for 601 s, the first N₂ signal was observed just after 2 s,indicative of the high CO₂ selectivity (520±200), which is in quite goodagreement with the predicted results using IAST.

It is noteworthy to mention that the CO₂ uptake (volumetric andgravimetric) at the partial pressure of interest for post-combustion(1.26 mmol/g and 0.84 mmol/g at 0.1 bar for single gas and CO₂/N₂:10/90mixture, respectively) is much lower for kag-MOF than the correspondingCO₂ uptake for the best MOF materials reported so far such as Mg-MOF-74,and SIFSIX-3-M. Nevertheless, the recorded density of CO₂ adsorbed phaseis one of the highest for MOFs (FIG. 17).

In addition to the efficiency of N₂ and CH₄ separation from CO₂,improving hydrothermal and chemical stability of MOFs in order toimplement them in real-world conditions is another primordial ongoingkey challenge for many MOFs. The thermal stability of the kag-MOF wasevaluated using powder X-ray diffraction (PXRD) and maintains itscrystallinity upon heating to temperatures up to 400° C. (FIG. 18). Thesame outstanding stability was observed after 24 h soaking in water,boiling water, at different pH (FIG. 5) and other organic solvents (FIG.19). All these results in addition to the rare hydrothermal nature ofsynthesis of kag-MOF confirm the exceptional thermal and chemicalstability of the kag-MOF.

Experimental Methods: Synthesis of Zn₅(HTet)₆(Tet)₃(OH⁻)₇

A solution containing Zn(NO₃)₃•6H₂O (29.7 mg, 0.1 mmol),Tetrazole-5-ethylester (32.8 mg, 0.2 mmol), 4 ml H₂O was prepared in aTeflon lined autoclave and heated to 160° C. for 24 h. Colorlesscrystals were harvested and air dried (Yield: 45%). Elemental Analysisfor calculated formula Zn₅(HTet)₆(Tet)₃(OH⁻)₇: C=11.04% (theo.: 10.08%),H=2.01% (2.07%), N=45.94% (47.02%).

Single Crystal X-Ray Diffraction

TABLE 1 Crystal data and structure refinement Empirical formulaC₉H₉N₃₆O₇Zn₅ Formula weight 1060.37 Temperature 150(2) K Wavelength1.54178 A Crystal system, space group Trigonal, P -3 1 c Unit celldimensions a = 12.478(1) A c = 12.708(1) Å Volume 1713.6(5) A{circumflexover ( )}3 Z, Calculated density 2, 2.055 Absorption coefficient 4.741mm⁻¹ F(000) 1042 Crystal size 0.340 × 0.205 × 0.069 mm Theta range fordata collection 4.09° to 66.83° Limiting indices −14 <= h <= 14, −14 <=k <= 14, −14 <= 1 <= 12 Reflections collected/unique 17668/969 [R(int) =0.0542] Completeness to theta = 66.83 94.5% Absorption correctionSemi-empirical from equivalents Max. and min. transmission 0.7528 and0.6451 Data/restraints/parameters 969/0/89 Goodness-of-fit onF{circumflex over ( )}2 1.054 Final R indices [I > 2sigma(I)] R₁ =0.0729, wR₂ = 0.2330 R indices (all data) R₁ = 0.0746, wR₂ = 0.2367Largest diff. peak and hole 1.860 and −2.248 e · A{circumflex over( )}⁻³

What is claimed is:
 1. A metal organic framework (MOF) compositioncomprising: one or more metal ions; and a plurality of organic ligands;wherein the one or more metal ions associate with the plurality oforganic ligands sufficient to form a MOF with kagomé (kag) topology. 2.The composition of claim 1, wherein the plurality of ligands includesone or more of tetrazole, triazole, derivatives of tetrazole, andderivatives of triazole.
 3. The composition of claim 1, wherein the oneor more metal ions includes one or more M²⁺ metal ions.
 4. Thecomposition of claim 1, wherein the one or more metal ions includes oneor more of Zn²⁺, Cu²⁺, Ni²⁺, and Co²⁺.
 5. The composition of claim 1,further comprising a solvent.
 6. The composition of claim 5, wherein thesolvent only includes water.
 7. The composition of claim 1, wherein apore size of the MOF is about 3.4 Å to about 4.8 Å.
 8. The compositionof claim 1, wherein the one or more metal ions includes Zn²⁺ and whereinthe plurality of ligands includes tetrazole.
 9. The composition of claim1, wherein the MOF includes Zn₅(HTet)₆(Tet)₃(OH⁻)₇.
 10. The compositionof claim 1, wherein the MOF has one or more of the followingcharacteristics: a channel diameter of about 3.6 Å to about 3.8 Å; a BETsurface area of about 211 m² g⁻¹; and a total free pore volume of about0.12 cm³ g⁻¹,
 11. The composition of claim 1, wherein the MOFcrystallizes in the hexagonal space group P63/mmc.
 12. A method ofmaking a metal organic framework (MOF) composition, the methodcomprising: contacting one or more metal ions with a plurality oforganic ligands in the presence of a solvent, sufficient to form a MOFwith kagomé (kag) topology.
 13. The method of claim 12, wherein the oneor more metal ions includes one or more M²⁺ metal ions.
 14. The methodof claim 12, wherein the one or more metal ions includes one or more ofZn²⁺, Cu²⁺, Ni²⁺, and Co²⁺.
 15. The method of claim 12, wherein theplurality of ligands includes N donor ligands.
 16. The method of claim12, wherein the plurality of ligands includes one or more of tetrazole,triazole, derivatives of tetrazole, and derivatives of triazole.
 17. Themethod of claim 12, wherein the solvent only includes water.
 18. Themethod of claim 12, wherein the one or more metal ions includesZn(NO₃)₃.6H₂O and wherein the plurality of ligands includestetrazole-5-ethylester.
 19. The method of claim 12, wherein thecontacting includes heating.
 20. The method of claim 12, wherein thecontact includes heating to about 160° C. for about 24 h.